FIG. 1 illustrates a schematic diagram of a conventional three-phase, 12 step (pulse) transformer rectifier power supply 10. The conventional 12 step transformer rectifier power supply 10 shown is designed to provide lower harmonic AC input current and relatively high power factor for DC outputs. However, these existing designs are deficient in that, secondary voltages produced are not inherently current limit protected, and separate circuits need to be employed to protect against primary transformer failure or secondary circuit electrical failures or short circuits.
From FIG. 1, a three phase input voltage, e.g., at 50 Hz, 60 Hz or 400 Hz can typically be applied to a three phase transformer with Delta and Wye secondaries adjusted in turns for equal voltage output to two 3-phase rectifiers with each rectified secondary being placed in parallel through balancing inductors to a common load, the voltage so derived is usable by electronic loads. The Wye transformer secondary voltage phase shift with respect to the corresponding Delta secondary is 15 degrees, and hence the ‘step’ input current waveform with 360degrees/30=12 steps or input current pulses. Similarly a 24 step(pulse) transformer would contain primary Delta-Wye interconnected windings and corresponding secondary winding yielding an input current waveform with 360 degrees/15=24 steps or input current pulses. Since this waveform has twice as many steps, it more closely approximates a sinusoidal input current and will yield 5-6% harmonic current. However since abrupt changes in the current are still present the EMI spectrum is large, and no overload protection is provided for the high power, or low impedance transformer, that is failure of a rectifier or shorted circuit on the load will require fuses and or circuit interrupt switches. In addition turn inrush current is not limited when an input switch 101 is activated (turned-on), unless a specific inrush current limiter is added.
FIG. 2 depicts a plot of an input current waveform 20 at any of the three line phase lines of the conventional 12 step (pulse) transformer rectifier power supply (e.g., a 48 Volt, 2 KW load) in an uncorrected power supply system that produces input harmonic currents exceeding 10% which is undesirable for most current power systems. Most military-grade power systems require less than 5% limits on harmonic current.
In FIG. 1, the supply 10 includes a three phase transformer with separate Delta and Wye secondaries, two full wave rectifiers connected trough balancing inductors to a common load. The source may be a three phase commercial power line such as the 110-125 volt 50-60 HZ or 400 HZ available from a generator. The common parallel secondary D.C. Voltage is then filtered by output capacitor Co and used by the electronic loads. The Delta transformer Primary to Delta transformer secondary voltage phase remains a zero phase shift while the Wye secondary phase shift with respect to the Delta primary is 30 degrees, after rectification and paralleling with supply loading the input ‘step’ or pulse current waveform is generated.
This output voltage from power supply is a suitable low voltage supply for any of a number of electronic equipment applications, such as computer systems, medical instrumentation, telephone switching systems, machine control systems, or other apparatus employing semiconductor devices or integrated circuitry or that requires supply voltages.
By way of example, however, the typical efficiency of such prior art power supplies may exceed 85% for 425 volt outputs and 80% for 48 volt outputs, but with relatively low power factors for reasons discussed herein with reference to FIG. 2.